Chapter 7: Random Access Memory

Every time your computer's processor performs an operation, it obtains instructions and stores data in memory. As the previous chapter explained, modern computers contain several types of memory, each in a different location within the system's logical structure.

This chapter concentrates on the Random Access Memory, or RAM, modules that store and forward commands to and from input and output devices such as the keyboard, the graphics display, and network links, and with permanent storage on disk drives, the BIOS, and removable flash drives. These modules are also called the computer's main memory or system memory.

First in the chapter is a very brief explanation of how these memory modules contribute to the computer's overall activity, then how to improve performance by adding memory, and finally how to confirm that the memory modules installed in your computer are working properly.

How Memory Works

The memory modules installed inside your computer contain RAM circuits that hold several hundred million memory cells. Each cell, made up of a capacitor and a transistor, represents one data bit (a capacitor is an electronic component that stores an electric charge; a transistor uses one electric charge or current to control the flow of another electric current). The memory circuit's control logic uses the transistor to read or change the charge on the capacitor. The charge state of the capacitor specifies the value of the bit assigned to that memory cell (on or off, 1 or 0, positive or negative).

The data bits are organized on each memory chip in an address system that allows the CPU to retrieve data from any address, thus Random Access Memory. Each bit in a computer's memory has a unique address, which is identified with a specific number. The bits are arranged in a rectangular grid of rows and columns, so the first part of the address identifies the row in which the bit is located, and the second part identities the column.

In order to preserve the memory's state, the memory controller must constantly refresh the charge on the capacitors. RAM that receives a refresh charge many times per second is called dynamic RAM, or DRAM.

The alternative to DRAM is static RAM, or SRAM. In SRAM, the circuit uses several transistors to set the state of each bit. Once the bit state is set, it does not change, as long as the transistors are continue to receive power, so there is no need for any refresh charge. The memory clears itself when the computer is turned off. Static ram is faster than DRAM, but it's also more expensive because it uses more transistors. The L1 and L2 cache in a CPU chip are normally SRAM because speedy access is the most important characteristic of cache memory, but it's less practical for the much larger main memory.

CROSS-REF 

See Chapter 6 for more about L1 and L2 cache memory.

In the earliest personal computers, the individual memory-integrated circuits were mounted directly on the motherboard, but as the amount of memory that a computer could use increased, it became more practical to package memory chips on separate smaller printed circuit boards, or modules, that could mount onto the motherboard through a card-edge connector and a socket. This modular approach made it possible to fit more memory into the same amount of space on the motherboard and to easily increase the size of the system memory by adding or replacing one or more modules.

All of the memory modules that are compatible with your computer's motherboard have the same physical dimensions, so it's easy to remove a module with a small capacity and replace it with a bigger one (bigger capacity, that is), up to the maximum amount of memory your computer can use. The smallest memory modules you're likely to find for a new computer contain 256MB of RAM. The capacity increases in multiples of 256MB, with 512MB, 1GB (1024MB), and 2GB the most common. If you're upgrading a computer with an older motherboard, you might also find modules with 64MB or 128MB (but if your computer is that old, it might be more cost-effective to simply replace it with a new one-memory modules for those old computers are often very expensive).